By Humeyra Pamuk and Pavel Polityuk WASHINGTON/KYIV (Reuters) -The United States warned on Monday there would be severe consequences if Russia used a nuclear weapon in Ukraine, while Russia took its claim that Kyiv was planning to deploy a “dirty bomb” to the United Nations. With Ukrainian forces advancing into Russian-occupied Kherson province, top Russian officials phoned Western counterparts on Sunday and Monday to tell them of Moscow’s suspicions. The foreign ministers of France, Britain and the United States rejected the allegations as “transparently false” and reaffirmed their support fo…